The background is presented herein only by way of example to multi-chip modules (MCMs), which are multi-material, multi-layered devices that are becoming one of the electronics packaging industry's most preferred components for a variety of aerospace, computer, military, and telecommunications applications. MCMs are replacing or reducing the complexity of printed circuit boards, thus enhancing product efficiency and reliability. MCMs present, however, new manufacturing obstacles because they require smaller vias and finer lines, and use a variety of new materials. Vias are discussed herein only by way of example and may take the form of complete through-holes or incomplete holes called blind vias.
MCMs and other multi-material, multi-layered electronic devices for packaging single chips such as ball grid arrays, pin grid arrays, etc; circuit boards; and hybrid and semiconductor microcircuits typically include separate component layers of metal and an organic dielectric and/or reinforcement materials. The standard metal component layer(s) may contain aluminum, copper, gold, molybdenum, nickel, palladium, platinum, silver, titanium, or tungsten, or combinations thereof. These layers typically have a depth or thickness of about 9-36 .mu.m (where 7.8.times.10.sup.-3 kg of metal equals a thickness of about 9 .mu.m), but may be thinner or as large as 72 .mu.m. A standard organic dielectric layer may include bismaleimide triazine (BT), cardboard, cyanate esters, epoxies, phenolics, polyimides, or polytetrafluorethylene (PTFE). These layers typically have a depth of about 50-400 .mu.m. A standard reinforcement component "layer" may include fiber matts or dispersed particles of aramid fibers, ceramics, glass, or Kevlar.TM. woven or dispersed into the organic dielectric layer to reinforce it. These reinforcements typically have a diameter or thickness of about 1-10 .mu.m. Stacks having several layers of metal, dielectric, and reinforcement material may be larger than 2 mm.
Traditional tools, punches, and production processes are designed for machining larger, less dense components. For example, well-known mechanical processes are either inadequate or prohibitively expensive for generating vias with diameters as small as 12 .mu.m. Even when miniaturization is not the chief issue, mechanical processes are still inadequate. For example, laminate circuit board applications are plagued by the wear of mechanical drills on the laminate and therefore entail the frequent and expensive sharpening or replacement of tools. Furthermore, conventional chemical or wet processes cannot be used to etch certain materials, such as Teflon.TM. dielectrics. Finally, electron milling, i.e., ion etching, is very expensive and too slow to process MCMs and most other electronic components.
Much work has been directed toward developing laser-based micromachining techniques to process these types of electronic materials. However, laser types, operating costs, and laser- and target material-specific operating parameters such as beam wavelength, power, and spot size vary widely.
Conventional Excimer lasers, for example, may generate laser output wavelengths from about 200 to 350 nm, but create poor quality multi-mode beam structures that simple lens elements cannot bring to a tight focus and, consequently, must be tailored by complex and expensive beam-shape controlling masks or apertures. Thus, an excimer laser beam cannot practically be of a high power density comparable to that achievable by the invention. These lasers are also generally limited to repetition rates of less than 200 Hz and are, therefore, too slow for deployment as a production tool for a variety of desirable applications. In addition, the high cost of excimer systems and their gases prevents their easy deployment and puts them beyond the reach of many small and mid-sized manufacturers. The halogen gases used in excimer laser processing chemically react with the components of the resonator and thereby cause degradation and frequent replacement. Moreover, halogen gases are materials that are hazardous, toxic, and damaging to the environment.
Conventional CO.sub.2 lasers, on the other hand, typically generate laser output wavelengths of about 10.6 .mu.m, creating spot sizes that are too large for conventionally desirable via processing. As compared with ultraviolet lasers, CO.sub.2 and other IR lasers have considerably longer pulse widths and much wider variance in the absorptivity of the organic materials and metals. These characteristics result in a destructively thermal, rather than photochemical, process.
The need for a beam-shape controlling mask in conjunction with an excimer laser limits the depth of cut per pulse one can achieve with an excimer laser beam, irrespective of whether the target is multi-layered. This is true after the excimer laser beam reaches a particular output power density that produces in the target material a so-called saturation depth of cut per pulse that cannot be increased with an increase in beam output power density. The mask method forces ablation of a large area rather than at a point.